1. Field of the Invention
The present invention relates to a fuel cell, and in particular to a fuel cell which runs on a liquid fuel and is capable of making full use of the fuel in a fuel reservoir.
2. Description of the Related Art
Fuel cells are devices for generating electric energy from hydrogen and oxygen, and are capable of providing high power generation efficiency. One of the main features of a fuel cell is that electrical power is generated through direct power generation, which is in contrast to conventional power generation methods where electrical power is generated via thermal or kinetic energy processes. Therefore, in a fuel cell, high efficiency power generation can be expected even in small-scale systems. Furthermore, a fuel cell is considered to be very environmentally friendly as it emits lesser amounts of nitrogen compounds and the like and generates less noise and vibration than conventional power generation methods. Since fuel cells can thus make effective use of the chemical energy of fuel, and have environmentally friendly characteristics, they are expected to serve as the energy supply systems of the 21st century. Fuel cells are attracting considerable attention as promising novel generation systems in a variety of applications ranging from large-scale power generation to small-scale power generation, including space technologies, automobiles, and portable devices. Thus, the technological development of the fuel cell for practical use is well under way.
Above all, polymer electrolyte fuel cells are characterized by lower operating temperatures and higher output densities as compared to the other types of fuel cells. Among various types of polymer electrolyte fuel cells, direct methanol fuel cells (DMFCs) have recently been gaining attention in particular. In DMFCs, an aqueous methanol solution (or pure methanol) serving as the fuel is supplied directly to the anode without any modification so that electrical power is generated through an electrochemical reaction between the aqueous methanol solution and oxygen. During this electrochemical reaction, carbon dioxide is discharged from the anode, and water is produced at the cathode as a reaction product. In comparison to hydrogen, aqueous methanol solution provides higher energy per unit volume, and has a lower risk of explosion or the like and is also suitable for storage. Thus, a DMFC is expected to be employed as a power source for automobiles, portable devices (cellular phones, notebook PCs, PDAs, MP3 players, digital cameras, and electronic dictionaries (books)), and the like.
In conventional fuel cells such as that disclosed in Japanese Patent Laid-Open Publication No. 2004-039293, the aqueous methanol solution to be supplied to the anode is delivered using a fuel supply unit such as a liquid pump. In the case of a so-called passive fuel cell where pumps and other driving sources are omitted as much as possible, however, the main body of the fuel cell has no driving source for delivering the fuel from a replaceable fuel reservoir (such as fuel cartridge) to a fuel chamber. In this instance, the fuel cartridge is removably connected to the fuel cell body. The fuel chamber is arranged next to the anode, and temporarily retains the fuel to be supplied to the anode by the action of natural convection, capillary force, or the like. The problem with this arrangement is that the fuel cartridge must be replaced when the fuel in the fuel cartridge and the fuel in the fuel chamber reach an equal concentration level, or when the fuel must be delivered from the fuel cartridge to the fuel chamber by the application of a spring force, gravity, or a squeezing force or the like imparted by the user.